Magneto resistance means electrical resistance or electrical resistivity which changes when a magnetic field is applied to a magnetic substance. Such magnetic resistance is classified into anisotropic magnetoresistance, giant magnetoresistance, tunneling magnetoresistance and colossal magnetoresistance, which all show differences in the theory and principle. A low-power micro magnetic field sensor can be produced by using a film of magnetic substance of nano thickness as the magnetoresistance material, because the characteristic of magnetoresistance is maintained even in a nano size shape. Also, not only it is possible to detect a ultra low magnetic field as the output can be converted into a voltage signal using the characteristic of electrical resistance which changes in accordance with the strength of the magnetic field but also it can be applied as a direct current (DC) and an alternating current (AC) broadband magnetic field sensor as the AC frequency characteristic is superior because it uses resistance change. Accordingly, magnetoresistance phenomenon is used for the head of a hard disk, magnetic field sensor, biosensor, compass, angle sensor, low frequency eddy current sensor for non-destructive inspection and high speed magnetoresistance RAM, etc.
The degree of change in resistance (ΔR) which occurs in accordance with change in the strength of magnetic field of magnetoresistance material is called magnetoresistance ratio (MR [%]) and is expressed in percentage as MR (%)=ΔR/R×100.
Anisotropic magnetoresistance (AMR) is a phenomenon which appears in a single magnetic layer. It has a characteristic of depending on the angle between the directions of current and magnetization and has a magnetoresistance ratio of about 2˜5% at ambient temperature. Therefore, the AMR phenomena can be separated two different effects such as anisotropic magnetoresistance effect (AME) and planar Hall effect (PHE), which are defined by the measurement geometry.
Giant magnetoresistance which has a structure of ferromagnet-metal-ferromagnet shows a characteristic of being dependant on the angle between the directions of magnetization of two ferromagnetic layers and independent on the direction of the current. That is, the resistance in the current changes due to spin scattering which depends on the directions of magnetization of the two ferromagnetic layers. The resistance is smallest when the magnetization directions of the two ferromagnetic layers are parallel with each other and is largest when the magnetization directions of the two ferromagnetic layers are antiparallel. In case of such a giant magnetoresistance, the magnetoresistance ratio is about 10˜50%.
Tunneling magnetoresistance which has a structure of ferromagnet-insulator-ferromagnet also shows a characteristic of being dependant on the angle between the directions of magnetization of two ferromagnetic layers and irrelevant to the direction of the current. That is to say, the resistance in the current which flows tunneling through the insulator between the two ferromagnetic layers changes due to spin scattering which depends on the directions of magnetization of the two ferromagnetic layers. The resistance is smallest when the magnetization directions of the two ferromagnetic layers are parallel with each other and is largest when the magnetization directions of the two ferromagnetic layers are antiparallel. In case of such tunneling magnetoresistance, material with magnetoresistance ratio higher than 500% has been already developed and magnetoresistance of 1000% or higher is expected.
In case a magnetic sensor is used as a magnetoresistance material, the output signal is proportional to the magnetoresistance ratio. Accordingly, in order to enhance the sensitivity by increasing the output signal characteristic which depends on the strength of the magnetic field, the magnetoresistance ratio should be large and linearity of the output signal for the strength of the magnetic field should be secured. However, in case of giant magnetoresistance material or tunneling magnetoresistance material, though it has a strong point of increasing the output signal because the magnetoresistance ratio is large, it has a weak point of having a bad linearity in the zero magnetic field range.
Accordingly, utilizing their superior signal characteristics, giant magnetoresistance material and tunneling magnetoresistance material are used for heads detecting hard disk signals or magnetoresistance RAMs for which no linearity is required. However, in case giant magnetoresistance material or tunneling magnetoresistance material is used as a magnetoresistance sensor, it has weaknesses that the structure of the magnetic sensor becomes complicated as the offset voltage of output signal should be removed while the specified magnetic field is applied because the area where linearity is secured is distributed across the predetermined magnetic field area and that the sensitivity is degraded due to the additional devices.
In the meantime, in case of anisotropic magnetoresistance material, though its magnetoresistance ratio is smaller than that of giant magnetoresistance material or tunneling magnetoresistance material, it has a superior linearity of output signal for the strength of magnetic field in the zero magnetic field range. Also, direction of current can be adjusted by changing the geometry structure of the magnetic sensor based on the principle of anisotropic magnetoresistance characteristic that the characteristic of output signal depends on the angle between directions of the current and magnetization and, accordingly, sensitivity which is a reference for measurement of magnetic field can be improved in principle by enhancing the characteristic of output signal of magnetic sensor through change in the geometry structure and improvement of characteristics of magnetoresistance ratio of the material.
Magnetic sensors which detect magnetic fields using such principle of anisotropic magnetoresistance phenomena are generally classified into anisotropic magnetoresistance effect (AME) sensors and planar Hall effect (PHE) sensors. In the AME sensor, the magnetoresistance itself changed by the magnetic field is measured as the directions of measuring the voltage and of the current are parallel with each other. In the PHE sensor, the direction of measuring the voltage and the direction of the current are perpendicular with each other.
U.S. Pat. No. 4,441,072, U.S. Pat. No. 4,533,572, U.S. Pat. No. 4,569,742, U.S. Pat. No. 4,681,812 U.S. Pat. No. 4,847,584 or U.S. Pat. No. 6,529,144 provide an anisotropic magnetoresistance sensor. WO2006/010014 provides an integrated magnetoresistive speed and direction sensor and WO 2001/57506 provides a magnetic sensor for a bio-sensor using a circular magnetic moment. However, the conventional magnetic sensor only provides a magnetic sensor only using either anisotropic magnetoresistance effect or a planar Hall effect.
A magnetic sensor which only uses either anisotropic magnetoresistance or planar Hall effect is using a single ferromagnetic layer. When a strong magnetic field is applied to such a magnetic sensor, it may cause reversal of output signal due to hysteretic behavior of magnetization. In order to supplement such a characteristic, a driving device is added which applies a strong magnetic field to the magnetic film layer when driving the sensor initially in order to stabilize the output signal by aligning magnetic domains in one direction.
Also, the range of magnetic field which can be measured is limited to the strength of anisotropy field of the magnetic layer when using a single ferromagnetic layer. For example, when NiFe film is used, the measurement range is limited to the range of magnetic field of about 5 Oe. Therefore, measurement of minute magnetic field changing under a magnetic field exceeding the predetermined strength (in case of NiFe, H>5 Oe) is impossible. Thus, in case an anisotropic magnetoresistance effect sensor made of a single layer of NiFe is used for a mobile phone to measure the earth magnetic field for the compass, it will be impossible to make the measurement of earth magnetic field because of the magnetic field of 5-10 Oe inside the equipment generated within the internal circuit and electronic components of the mobile phone.
The characteristic of signal of anisotropic magnetoresistance effect sensor is decreased if the resistance of the sensor is reduced. That is, there is a limit in using an anisotropic magnetoresistance effect sensor as a bio-sensor for measurement of micro magnetic beads since it has a weak point that the signal is decreased when the size of the sensor is reduced in order to measure micro-sized magnetic beads. Accordingly, a magnetic sensor which uses planar Hall effect in which output signal does not have anything to do with the size of the sensor is used as a bio-sensor for measurement of micro magnetic bead.
As the anisotropic magnetoresistance or planar Hall effects are dependent upon the directions of current and of magnetic domain, both effects can be adjusted by changing the shape of the magnetic sensor. In case of the magnetic sensor which uses both effects of anisotropic magnetoresistance or planar Hall, the bead detection capability of the bio-sensor used to measure micro magnetic beads can be enhanced since output signal characteristic can be improved. Accordingly, it is produced as a micro size and low-power magnetic sensor and used as the magnetic sensor or compass sensor for mobile phones or electronic apparatus as the range of magnetic field measurement can be adjusted and the characteristic of magnetic hysteresis can be eliminated when exchange coupling is combined with ferromagnetic layer.